Aircraft Fuel Burn: Factors and Calculations

The Critical Nature of Fuel Planning

Running out of fuel is one of the most preventable emergencies in aviation, yet it remains a leading cause of engine failures. Proper fuel planning goes beyond just knowing your aircraft's average gallons per hour (GPH) burn rate; it requires a nuanced understanding of how different flight phases affect consumption.

Factors Affecting Fuel Consumption

Several variables impact how much fuel your engine consumes:

• Power Setting: The RPM and manifold pressure you select dictate fuel flow. A high-speed cruise burns significantly more fuel than an economy cruise.
• Altitude: Generally, normally aspirated engines burn less fuel at higher altitudes because the thinner air requires leaning the mixture to maintain the correct air-to-fuel ratio.
• Weight: A heavier aircraft requires a higher angle of attack to maintain altitude, increasing drag and requiring more power (and fuel) to maintain airspeed.
• Winds: While winds don't change your burn rate per hour, headwinds dramatically increase your total fuel required for a trip by extending your time en route.

Phases of Flight

A standard fuel calculation must account for the different phases of flight. Takeoff and climb require full rich mixtures and high power settings, resulting in the highest burn rates. Cruise is typically leaned out for best economy or best power. Descent burns the least fuel. When planning, always pad your climb fuel and calculate cruise fuel based on conservative Ground Speed estimates.

Reserve Requirements

VFR regulations require enough fuel to fly to your destination plus 30 minutes at normal cruising speed during the day (45 minutes at night). IFR regulations are stricter. However, personal minimums should often exceed legal minimums—many pilots insist on landing with at least one hour of fuel remaining in the tanks.